Answer:
He still needs to fill 79.43% of the tank.
Explanation:
How much of the tank he filled?
1 3/4 gallons
As a decimal number, this is:

Conversion to liters:
1 gallon has 3.785 liters.
So, for 1.75 gallons, we have 1.75*3.785 = 6.62 liters.
The fish tank holds 32.176 liters.
So, he still needs to fill 32.176 - 6.62 = 25.556 liters.
What percentage of the tank does he still need to fill? ​
We divide what need to be filled by the total capacity, and multiply by 100. So

He still needs to fill 79.43% of the tank.